Patent Document 1, for example, discloses a power control technology for converting a direct-current power supply, which is obtained by rectifying and smoothing a battery or an alternating current, into a power required by a discharge lamp, in order to stably light the discharge lamp. In this technology, a lamp current command value that is supplied to the discharge lamp is set based on a lamp voltage detection value, and the control variable of a power converter circuit configuring a discharge lamp lighting circuit is regulated based on the calculated difference between the control variable and a lamp current detection value.
A technology for suppressing a ground-fault current is shown in FIG. 15 (see Patent Document 2, for example). This technology is configured to alternate a direct-current output of a DC-DC converter circuit 11 by means of an inverter circuit 12 and supply thus obtained alternating-current power to a discharge lamp. The DC-DC converter circuit 11 is a power converter circuit with a function for regulating the power supplied to the discharge lamp. One end of an output terminal of the DC-DC converter circuit 11 is connected to one end of an input terminal of the inverter circuit 12 via a current sensing resistor R. A lamp current is detected using the current sensing resistor R. Moreover, the one end of the input of the inverter circuit 12 to which one end of the current sensing resistor R is connected is used as a circuit ground (earthing) terminal, and a voltage signal generated at the other end of the current sensing resistor R is used as a current detection signal and compared with a command value. Then, a PWM signal for controlling a switching element Q1 configuring the DC-DC converter circuit 11 is regulated by using a feedback circuit. With the configuration described above, even when a ground fault occurs at an output terminal of the discharge lamp lighting device as shown by a dashed line in FIG. 15, a ground-fault current flows through a closed circuit that extends from a ground-fault point, via a terminal on the grounded side of the power supply and definitely via the current sensing resistor R, to the output terminal of the DC-DC converter circuit 11. For this reason, is it possible to control the power including the ground-fault current, and also suppress the ground-fault current.
As shown in FIG. 16, this technology has a configuration in which two current sensing resistors R1, R2 are connected, a current to be supplied to the discharge lamp is detected in a series circuit of the resistors R1, R2, and the ground-fault current flows through the resistor R1. At the time of normal lighting, the voltage drop amount at each end of the series circuit of the resistors R1, R2 is the lamp current detection value. This value is detected by a difference amplifier A. Then the difference between this value and a command signal Vk is calculated, and the PWM signal for controlling the switching element Q1 is regulated. The ground-fault current flows only through the resistor R1. Since the voltage drop amount that results from the ground-fault current at the resistor R1 is input to the difference amplifier A through the resistor R2, it is possible to limit the ground-fault current. However, the disadvantage is that the ground-fault current becomes greater than the lamp current obtained at the time of lighting, because the sensing resistors become smaller than at the time of the normal lighting.
There is the following technology for protecting a discharge lamp lighting device from a ground-fault current (see Patent Document 3, for example). In this technology, current sensing resistors are connected as in the configuration shown in FIG. 15. If overcurrent flows even when a predetermined amount of time elapses after starting the discharge lamp lighting device, it is then determined that overcurrent resulting from a ground fault flows, and accordingly the discharge lamp lighting device is stopped, whereby this device is protected.
There is also the following technology for protecting a discharge lamp lighting device from a ground-fault current (see Patent Document 4, for example). This technology focuses on the fact that an output voltage increases up to a predetermined no-load secondary voltage that is higher than when the discharge lamp is in a steady lighting state, because a load current does not flow when the discharge lamp is in an unlit state. In the case where an output voltage or output current of the discharge lamp lighting device, which is generated when the discharge lamp is in the unlit state immediately after the discharge lamp lighting device starts operating, deviates from an assumed range, it is determined that a ground fault occurs at an output terminal, and the discharge lamp lighting device is stopped and accordingly protected. The case where the output voltage deviates from the assumed range means that the output voltage does not increase to a predetermined voltage within a predetermined time period when the output voltage supposedly increases to a no-load voltage. The case where the output current deviates from the assumed range means that a predetermined value or more of the load current does not flow during a predetermined time period when the output voltage supposedly increases to the no-load voltage.